The atomic energy agency, Rosatom, said in a statement that the ADE-5 reactor at the Siberian Chemical Plant in Seversk stopped operation and workers will begin removing remaining uranium fuel. It will take several years to dismantle the reactor's technical equipment.
The plant's first reactor was shut down on April 20. The last plutonium-producing reactor, in the city of Zheleznogorsk, is expected to be shuttered by 2010.
Located in secret cities, the plants were part of the Soviet Union's nuclear weapons complex and produced weapons-grade plutonium over the course of 50 years. But in the early years after the Soviet breakup, the Defense Ministry stopped buying the plutonium.
The United States pushed for years to close down the plants, but they produced electricity and heat for nearby cities as a byproduct of their operations and the Russians did not want to leave Siberian cities without power before coal-fired replacement plants were built.
The United States committed $926 million to help build the fossil fuel plants, along with donations from Britain, Canada and other nations.
The design of the Seversk and Zheleznogorsk reactors -- similar to the Chernobyl reactor that exploded in 1986 -- also raised fears of accidents.
The plants together produced more than 1 ton of plutonium annually, according to the National Nuclear Security Administration, a U.S. agency that coordinates nonproliferation programs.
The United States has also funded efforts to help Russia pay for construction of a plant to turn the stockpiled plutonium into a mixed oxide nuclear fuel and for research into a more advanced reactor that could speed up the process of disposing of plutonium.
The United States -- which has closed all its 14 plutonium-production facilities -- is believed to have about 100 tons of weapons-grade plutonium stockpiled and Russia about 140 tons.
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